Oil recovery sensor

ABSTRACT

The present disclosure relates to systems and methods for measuring oil/water content in oil-water mixtures, regardless of the salinity of the mixture. The oil content is measured using a dielectric sensor. It is determined whether the oil content is above or below a threshold. If the oil content is above the threshold, the oil content is reported using the measurement from the dielectric sensor. If the oil content is below the threshold, the oil content is reported using the measurement from the eddy current sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/678,407, filed May 31, 2018, the disclosure of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under D01 Award E17PC00011, BMI Contract No. CON00026017 awarded by the Bureau of Safety and Environmental Enforcement of the United States Department of the Interior. The government has certain rights in the invention.

BACKGROUND

The present disclosure relates to systems and methods for determining the oil/water content of oil-water mixtures. This is done by using a dielectric sensor or an eddy current sensor.

According to the U.S. Department of Energy, 1.3 million gallons (4.9 million liters) of petroleum are spilled into U.S. waters from vessels and pipelines in a typical year. A major oil spill could easily double that amount. Generally, mechanical surface skimmers remove oil and oil-water mixtures from surface water.

Present systems for measuring oil content in oil-water mixtures encounter difficulties when the oil-water mixture has high electrical conductivity, as can occur with seawater or any highly saline water found in industrial or oil and gas applications. It would be desirable to provide systems and methods that address these difficulties.

BRIEF DESCRIPTION

The present disclosure provides systems and methods for measuring the oil/water content of oil-water mixtures, regardless of the salinity of the water in the mixture. Briefly, two different sensors are used, a dielectric sensor and an eddy current sensor. The dielectric sensor is used if the oil content is above a threshold value, which may be the transition point between oil-in-water and water-in-oil mixtures or emulsions, and the eddy current sensor is used if the oil content is below the threshold value. Computer programs and systems for determining which sensor measurement is more accurate are also described herein.

In accordance with one aspect of the present application, systems for measuring the oil content of a fluid are disclosed. The system may include a dielectric sensor, an eddy current sensor, and one or more processors. The dielectric sensor comprises a first resonance circuit formed by a first capacitor, a first inductor, and a pair of electrodes adapted to be attached to an associated cavity through which the fluid can flow. The dielectric sensor also comprises a first standing wave ratio (SWR) analyzer configured to measure a resonance frequency of the first resonance circuit. The eddy current sensor comprises a second resonance circuit formed by a second capacitor and a second inductor. The second inductor is configured to create a magnetic field within the associated cavity. The eddy current sensor also comprises a second SWR analyzer configured to measure a height of a peak of a resonance frequency of the second resonance circuit. The one or more processors are configured to: determine if the oil content is above or below a threshold; and if the oil content is above the threshold, measure the oil content using the dielectric sensor; and if the oil content is below the threshold, measure the oil content using the eddy current sensor. As described here, the two sensors can be attached to an existing cavity and then used, or a cavity can be provided.

The threshold may be an oil content of 70 percent. The threshold may alternatively be an oil content corresponding to a resonant frequency that allows for distinction between oil-in-water mixtures and water-in-oil mixtures. In some embodiments, the threshold is set based on the oil content (desired for the particular application). In some implementations, the operating frequency of the second SWR analyzer is selected based on: (i) a radius of a pipe of the sensor cavity, and (ii) a conductivity of the fluid. The second inductor may be a coil wound around a pipe of the sensor cavity. The second inductor is generally not in direct physical contact with the fluid passing through the sensor cavity.

In some implementations, the one or more processors are further configured to make the determination of whether the oil content is above or below the threshold by using the dielectric sensor. In some implementations, the eddy current sensor is configured to account for water salinity by calibration of the eddy current sensor performed with water having a given salinity. In some implementations, the eddy current sensor is configured to account for water salinity by manual entry of a salinity value. In some implementations, the first inductor, the first capacitor, and the sensor cavity are all connected in parallel.

In accordance with another aspect of the present disclosure, methods for measuring the oil content of a fluid in a cavity is disclosed. The method may include: using a dielectric sensor, measuring the oil content of the fluid; and using one or more processors to determine whether the oil content is below a threshold; and in response to the determination that the oil content is below the threshold, using an eddy current sensor to measure and report the oil content of the fluid.

The methods may further include: using the one or more processors, determining that the oil content is above the threshold; and in response to the determination that the oil content is above the threshold, using a dielectric sensor to measure and report the oil content of the fluid. In some implementations, the threshold is an oil content of 70 percent. In some implementations, the threshold is an oil content that allows for distinction between oil-in-water and water-in-oil mixtures. In some implementations, the eddy current sensor comprises: a resonance circuit formed by a capacitor, an inductor and the cavity, wherein the inductor is configured to expose the fluid of the sensor cavity to a magnetic field; and a standing wave ratio (SWR) analyzer configured to measure a height of a peak of a resonance frequency of the resonance circuit. The inductor can be in the form of a coil wound around a pipe of the sensor cavity.

In some implementations, the determination that the oil content is below the threshold is made based on a measurement from a dielectric sensor. In some implementations, the methods further include accounting for water salinity by calibrating the eddy current sensor using water of a given salinity. In some implementations, the method further includes accounting for water salinity by manually entering a salinity value into the eddy current sensor or the processor(s).

In accordance with another aspect, other systems for measuring oil content of a fluid are also disclosed. The system may include a dielectric sensor, comprising: a first resonance circuit formed by a first capacitor, a first inductor, and a sensor cavity through which the fluid can pass; and a first standing wave ratio (SWR) analyzer configured to measure a resonance frequency of the first resonance circuit. The system may further include an eddy current sensor, comprising: a second resonance circuit formed by a second capacitor, and a second inductor, wherein the second inductor is configured to expose the sensor cavity to a magnetic field; and a second SWR analyzer configured to measure a height of a peak of a resonance frequency of the second resonance circuit. The system may further include: at least one processor; and at least one memory including computer program code. The at least one memory and the computer program code may be configured to, with the at least one processor, cause the system to: determine if the oil content is above or below a threshold; if the oil content is above the threshold, measure and report the oil content using the dielectric sensor; and if the oil content is below the threshold, measure and report the oil content using the eddy current sensor.

Advantageously, the systems and methods described herein have the ability to accurately monitor and report oil-water percentages from 0 to 100% regardless of water salinity. By way of comparison, dielectric sensors alone cannot work with high electrical conductivity mixtures, for instance seawater-based mixtures that contain more than ˜30% of sea water. The disclosed approaches work with water of any salinity that is relatively stable (i.e. does not change quickly over time). This includes fresh water, and seawater from different seas, even if very concentrated.

Advantageously, in the approaches described herein, the response is independent of oil-water dispersion, including mixtures that are not homogeneous, homogenous, or an emulsion. In particular, the sensor system can provide accurate results whether the oil-water mixture is an oil-in-water mixture or a water-in-oil mixture.

The sensing system can also operate with an open pipe of any size serving as the sensor cavity. In addition, there is no need to install a flow conditioning device upstream or downstream of the sensor(s). For instance, there is no need for a homogenizer, which generally are not a good option for oil skimming operations as they become quickly clogged during operations and increase pressure drop.

The sensor system measurement can also be carried out at any pressure. The sensor system can also reliably detect the sensor cavity being empty. Metal electrodes do not need to make physical contact with the oil-water mixture to be tested. Advantageously, the sensor uses very minimal power. The measurement can be realized with as little as 0.1 mW of power. The sensor system can be certified as meeting ATEX Level 2 criteria (for use in potentially explosive atmospheres).

These and other non-limiting aspects of the present disclosure are described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same.

FIG. 1 illustrates a dielectric sensor.

FIG. 2 illustrates an eddy current sensor.

FIG. 3 shows components of an algorithm for calculation of oil/water content based on combined dielectric and eddy current measurements.

FIG. 4 is a diagram of an exemplary embodiment of a sensor system that includes SWE analyzers and a single-board computer.

FIG. 5 is a graph showing the response of a dielectric sensor measured as resonance frequency versus oil content at different mixer speeds. The y-axis is frequency in MHz, and runs from 34 to 42 at increments of 1. The x-axis is oil content in vol %, and runs from 0 to 100 at increments of 20. The dotted line indicates the frequency response for an empty pipe.

FIG. 6 is a graph showing the response of an eddy current sensor measured as peak resonance frequency height versus oil content at different mixer speeds. The resonance frequency was 21.72 MHz. The y-axis is peak height in decibels (dB), and runs from 3 to 30 at increments of 3. The x-axis is oil content in vol %, and runs from 0 to 100 at increments of 20. The dotted line indicates the peak height for an empty pipe.

FIG. 7 is a graph showing the response of an eddy current sensor measured as peak resonance frequency height versus oil content at different mixer speeds and different salinities (2 wt % and 4 wt %). The resonance frequency was 21.72 MHz. The y-axis is peak height in decibels (dB), and runs from 3 to 27 at increments of 3. The x-axis is oil content in vol %, and runs from 0 to 100 at increments of 20.

FIG. 8 shows the comparison between the measured oil fraction (based on algorithms described herein) and the true oil fraction, for showing the accuracy of the algorithm and sensor system. The y-axis is the measured oil fraction in vol %, and runs from 0 to 100 at increments of 20. The x-axis is the true oil fraction in vol %, and runs from 0 to 100 at increments of 20.

FIG. 9 shows an embodiment including an oil recovery sensor installed in a U-trap arrangement which ensures that the sensor cavity remains air-free.

DETAILED DESCRIPTION

The present disclosure may be understood more readily by reference to the following detailed description of desired embodiments and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings.

Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.

The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

The term “comprising” is used herein as requiring the presence of the named components/steps and allowing the presence of other components/steps. The term “comprising” should be construed to include the term “consisting of”, which allows the presence of only the named components/steps.

Numerical values should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of “from 2 grams to 10 grams” is inclusive of the endpoints, 2 grams and 10 grams, and all the intermediate values). The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and/or values.

The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context. When used in the context of a range, the modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the range of “from about 2 to about 10” also discloses the range “from 2 to 10.” The term “about” may refer to plus or minus 10% of the indicated number. For example, “about 10%” may indicate a range of 9% to 11%, and “about 1” may mean from 0.9-1.1.

The present application is related to oil-water mixtures. It should be understood that the oil content plus the water content generally equals 100% (solutes and other materials in the fluid not being considered).

A dielectric sensor may be used to measure water content in oil-water mixtures, gas-water mixtures, and moisture levels in solids. The principle of measurement is related to the large value of water's relative dielectric constant (permittivity), which is about 80. This value is much greater than the dielectric constant of gases (close to 1), organic liquids (like oils and crudes) as well as solids (below about 10). Dielectric sensors are generally constructed as capacitors that contain a cavity that is filled with the oil-water mixture. The cavity can be either a flow through device (e.g. a tube) or a batch device (e.g. a vessel or tank). The dielectric sensor detects changes in electrical capacitance caused by different water content of the fluid in the cavity. Such changes are detected by a direct capacitance measurement, or often by a detection of a frequency shift of a resonance circuit which includes the cavity itself.

However, the dielectric measurement is not reliable when the mixture being measured is highly electrically conductive, for instance seawater or any highly saline water found in industrial or oil and gas applications. The high conductivity of the mixture can be represented in the dielectric sensor as a low resistance connected in parallel with the cavity's capacitance which effectively shortens the cavity. This effect cannot be solved by modifying the shape or size of the cavity in which the fluid/mixture is measured, since the relative contribution of the cavity's capacitance and resistance is geometry-independent. Theoretically, the relative resistance contribution can be reduced by increasing the frequency of the dielectric measurement, since the capacitance contribution is increased at higher frequencies, while the resistive contribution remains constant. However, a frequency increase causes reduction of skin depth, which determines the extent of penetration of electromagnetic waves into the tested mixture. This decrease in skin depth penetration makes a sensor sensitive only to areas close to its electrodes, which may be a small fraction of overall sensor volume, especially for larger sensors. Due to the high salinity of typical seawater, the conductivity effect is severe enough to prevent successful development of dielectric sensors that by themselves are effective in marine environments. In the case of oil-water sensors operating with seawater, dielectric sensors are capable of measuring oil content only for low conductivity, water-in-oil mixtures/emulsions that contain a minimum of 60-70% oil.

The systems and methods described herein relate to sensor configurations capable of measuring the oil or water content of oil-water mixtures. The measurements are accurate even in cases where the water contains large amounts of salt, for example seawater. Very generally, measurement are made using: (1) a dielectric measurement, and (2) an eddy current loss measurement. The dielectric measurement may be used for high oil content mixtures/emulsions that are of the water-in-oil type, and therefore have relatively low electrical conductivity. For this type of mixture/emulsion, the dielectric sensor provides a reliable measurement for oil content. On the other hand, the eddy current measurement is used for low oil content mixtures/emulsions that are of the oil-in-water type and have high electrical conductivity. For this type of mixture/emulsion, the eddy current measurement provides a reliable measurement for oil content. The measurements can be compared to a reference table that accounts for oil content and salinity, etc. Using both sensors in the implementations as described herein allows for unambiguous and accurate measurement of oil or water content for a broad range of oil-water mixtures including mixtures of crude oils and/or mixtures with saline water.

The systems and methods described herein provide a reliable oil content measurement for a broad range of mixtures ranging from including pure saline water to pure oil. In one aspect, the device works at different water salinities and for different oil and crude types, and is insensitive to oil-water dispersion state. The sensors can be used in any application where oil-water mixtures need to be evaluated for oil content or water cut. One application of the sensor systems described herein is for offshore applications, for instance for evaluation of efficiency of oil recovery during spill clean-up operations.

The sensors can be operated at relatively high operating frequencies, from 1 MHz up to 1 GHz, which minimizes concerns related to electrode polarization. The proposed sensors also provide a relatively uniform sensitivity across a pipe cross section, which allows for use of open pipes with a broad diameter range, without any mixing or homogenizing devices while still obtaining accurate measurements.

FIG. 1 presents a design of a dielectric sensor 100, which is used in some embodiments of the present disclosure. The sensor is installed on a cavity 110 that contains the fluid whose oil/water content will be measured. The cavity itself can be considered part of the sensor system. The cavity is illustrated here as a pipe or tube through which fluid can flow. The cavity can be made of any suitable material, for example acrylic or PVC. The cavity may have any desired diameter. In experiments reported further herein, the cavity has a diameter of 3 inches. A pair of electrodes 120 is attached to the cavity 110. The electrodes are made of a suitable metal. The electrodes are usually attached to the exterior of the cavity 110. Together, the cavity and electrodes act as a capacitor, whose capacitance will change depending on the fluid in the cavity that is being measured.

The dielectric sensor 100 also includes a first capacitor 130 and a first inductor 140. The first capacitor may have a capacitance of about 10 picofarads (pF) to about 100 pF. The first inductor may have an inductance of about 0.1 microhenrys (μH) to about 2 μH. The electrodes 120, the first capacitor 130, and the first inductor 140 are connected to each other in parallel, i.e. in a parallel circuit. Together, these components form a first resonance circuit 150.

A secondary pickup coil 155 is installed proximate the first inductor 140, and connected to a first radio frequency Standing Wave Ratio (SWR) analyzer 160. The first SWR analyzer is used to identify the peak resonance frequency of the first resonance circuit. Examples of suitable SWR analyzers include the AA-170 and AA-1400, both produced by Rig Expert. If desired, a different device or method can be employed to measure the resonance peak. For instance, a frequency counter can be used to determine the resonance frequency. The distance between the first inductor and the secondary pickup coil can be adjusted to optimize the sharpness of the resonance peak. The SWR analyzer will scan an operating frequency range, desirably near the expected resonance frequency. The resonance frequency, defined by a maximum peak, is recorded for the oil-water mixture.

The sensor systems of the present disclosure also include an eddy current sensor. An eddy current measurement is effective for measuring the oil/water content in oil-in-water mixtures, while free of the electrical conductivity measurement drawbacks present in the dielectric sensor. The eddy current effect is observed in all conductive materials that are exposed to changing magnetic fields. Eddy currents are electrical currents that cause two effects: (1) they have an orientation and intensity that tends to cancel the external magnetic field that generates them, and (2) they cause energy losses due to heat generation in the conductive media. In fact, the eddy currents are responsible for the finite skin depth penetration of electromagnetic waves in conductive media. Energy losses due to eddy currents allow for measurement of this effect by means of a resonance circuit. If the inductor generating the eddy current effect is a part of the resonance circuit, eddy current energy losses in the tested material will cause losses in the resonance circuit. These losses will cause broadening and height reduction of a resonance peak, both easily measurable effects.

FIG. 2 illustrates an eddy current sensor 200. A second inductor 240 is attached to the cavity 110 in a manner that exposes the oil-water mixture within the cavity to a magnetic field. Desirably, the second inductor 240 is in the form of a coil wound around the cavity 110, as this ensures the most uniform sensitivity across the entire cross section of the cavity. Preferably, the coils of the second inductor are on the exterior of the cavity, and are not in direct electrical contact with the tested fluid within the cavity, since this would introduce direct conductivity effects and result in additional measuring problems. The coils of the inductors described in the present disclosure can be made from any suitable conductive metal, copper being the most suitable.

The second inductor 240 is connected to a second capacitor 230 and forms a second resonance circuit 250. The second inductor and the second capacitor are arranged in series with each other. The second capacitor 230 may have a capacitance of about 50 to about 700 pF, including from about 100 pF to about 300 pF. The second inductor may have an inductance of about 0.1 microhenrys (μH) to about 2 μH.

A secondary pickup coil 255 is installed proximate the second inductor 240, and connected to a second radio frequency Standing Wave Ratio (SWR) analyzer 260. This configuration for the eddy current sensor has a resonance frequency of about 21.72 megahertz (MHz). The second SWR analyzer is used to identify the magnitude of the resonance peak, which correlates with the oil/water content of the fluid within the cavity. Again, examples of suitable SWR analyzers include the AA-170 and AA-1400, both produced by Rig Expert. The distance between the second inductor and the secondary pickup coil can be adjusted to optimize their coupling and the sharpness of the resonance peak.

It has been found that the measurement of the dielectric sensor is most accurate for oil-water mixtures having an oil content above a threshold value, while the measurement of the eddy current sensor is most accurate for oil-water mixtures having an oil content below the threshold value. In one embodiment, the threshold value can be an oil content of 70 percent. In other embodiments, the threshold value can be an oil content corresponding to a resonance frequency that allows for distinction between oil-in-water mixtures and water-in-oil mixtures (this resonance frequency may vary depending on the values of the capacitor and inductor used in the dielectric sensor). Thus, a system using both types of sensors is expected to be most accurate over the entire range of possible oil/water values. The measurements made by the respective sensor can be compared to reference tables to determine the oil content.

A system using both types of measurements can be built with the cavity in the form of a single pipe or tube through which the oil-water mixture flows, with the two sensors being mounted on the single pipe and spaced apart from each other. The two measurements (by the dielectric sensor and the eddy current sensor) can be performed simultaneously or in short succession. It is expected that two simultaneous and continuous measurements will be possible with proper selection of operating frequencies that do not overlap, including harmonics overlap. Both measurements require very small power; the SWR analyzer used in both sensors has an output power of −10 dBm, which is equivalent to 0.1 mW.

FIG. 3 schematically illustrates an algorithm that can be used to report an accurate measurement of the oil/water content. The first part 310 of the algorithm identifies what type of mixture or emulsion is filling the sensor cavity. This can be determined by the resonance frequency obtained by the dielectric sensor. For instance, an oil-in-water emulsion is identified if the resonance frequency is less than a given value (which is affected by the capacitor and inductor values), and a water-in-oil emulsion is identified if the resonance frequency is above this value. This determines whether the reported oil/water content is based on the measurement from the dielectric sensor or the eddy current sensor. If the oil-water mixture is a water-in-oil emulsion, the measurement from the dielectric sensor 320 is reported. If the oil-water mixture is an oil-in-water emulsion, the measurement from the eddy current sensor 330 is reported.

FIG. 4 is a block diagram illustrating an exemplary embodiment of a sensor system of the present disclosure. The sensor system 400 includes components that are proximate the sensor cavity 410, electronics 412 for processing the measurements made by the sensors, and external controls 414. The components proximate the sensor cavity 410 include the dielectric sensor 420, the eddy current detector 430, and a temperature sensor 460 that is used to account for changes in water conductivity due to temperature. As described above, the components of the dielectric sensor 420 and the eddy current detector 430 are external to the sensor cavity, and do not need to contact the oil-water mixture that is present within the sensor cavity. The electronics 412 include the first SWR analyzer 425 for the dielectric sensor 420 and the second SWR analyzer 435 for the eddy current detector 430. A single board computer 440 is used to control the SWR analyzers, carry out algorithm calculations, and handle input and output operations. For example, the single board computer can compare the sensor measurements to reference tables or databases that identify the oil/water content based on the measurements made by the dielectric sensor and/or the eddy current sensor.

The SWR analyzer(s) can scan a preselected frequency range anywhere from a fraction of a megahertz up to tens, or for some units, hundreds of megahertz. SWR analyzers are available as handheld instruments with a simple keyboard and a display. Selected SWR analyzers are intended to be imbedded in larger instruments, and are constructed as a single electronic board without peripherals. These SWR analyzers use serial or USB ports for external communication and control. The output power of SWR analyzers is very small, on the order of milliwatts, which is sufficient for the present application.

In some embodiments, two RigExpert AA-30 ZERO single-board analyzers are used for the sensors. A BeagleBone Black open-source single-board computer can be used to control the SWR analyzers, carry out algorithm calculations, and handle input and output operations. The BeagleBone Black provides direct and simultaneous control of two AA-30 analyzers plus one temperature sensor, on-board storage of oil fraction data, and hosting of a website used for user interface.

For the dielectric sensor, operating frequencies below 10 MHz should be avoided because of electrode and oil-water interface polarization effects. Generally, any operating frequency in the range of about 10 MHz to about 300 MHz can be used. Other operating frequency ranges include about 40 MHz to about 270 MHz, and about 20 MHz to about 24 MHz. Skin depth consideration does not significantly affect the dielectric measurement, due to very low conductivities of water-in-oil mixtures.

For the eddy current sensor, operating frequencies below 10 MHz should also be avoided because of electrode and oil-water interface polarization effects. The operating frequency should be selected to provide effective dynamic range of the eddy current measurement. Two frequency selection methods can be used. First, the operating frequency is selected to be the maximum frequency, which allows for reliable eddy current height measurement for pure water with maximum salinity expected for a given sensor and electronic implementation. Second, the operating frequency is selected to provide a pure water skin depth penetration close to a radius of the sensor cavity. This can be determined according to the following Equation (1): f(MHz)=2500/(R ²σ)  (1) where R is the radius of the sensor cavity in cm, and a is the conductivity of the fluid in S/m. For instance, for seawater with conductivity of 5 S/m and a pipe that is 4 inches in diameter, the operating frequency of the order of 19 MHz may be appropriate. In some particular embodiments, the operating frequency for the eddy current sensor is from about 15 MHz to about 16 MHz.

Overlap between the operating frequencies of the dielectric sensor and the eddy current sensor, including their harmonics, should be avoided to prevent signal interference.

The physics of the eddy current effect can be affected by the salinity of the water. Thus, various embodiments described herein account for water salinity. This can be realized, for example, by calibration of the sensor system performed with pure water having a given salinity, or by directly entering the salinity value into the sensor. This permits the appropriate reference tables/databases to be used to identify the oil/water content based on the sensor measurements.

If there are no significant interactions between the oil and water, the electrical conductivity of the oil-water mixture (C_(Mixture)) can be expressed as the product of the pure saline water electrical conductivity (C_(Water)) and a geometrical factor (g), which accounts for presence of oil in the water, as shown in Equation (2) below. C _(Mixture) =C _(Water) ×g  (2)

The pure saline water conductivity (C_(Water)) is a function of water salinity and temperature, and its value can be calculated using published correlations. The geometrical factor (g) is a function of oil volume fraction, and oil-water dispersion. Experiments carried out with different oil types, various oil-in-water mixtures and emulsions, and frequencies in megahertz range (10-50 MHz) indicate that equation 1 above can be approximately solved by the following Equation (3).

$\begin{matrix} {{{Oil}\mspace{20mu}{fraction}}\; \approx {A\left( {1 - \frac{c_{Mixture}}{c_{Water}}} \right)}} & (3) \end{matrix}$ where A is a constant, and is between 60% and 70%. The mixture conductivity (C_(Mixture)), measured by the eddy current sensor, is a monotonic function of the eddy peak height, which can be determined by suitable sensor calibration procedure. Importantly, the oil fraction measurement based on Equation (3) is not sensitive to the homogeneity of the oil-water dispersion, and can be used with both coarse mixtures and stable emulsions.

In some embodiments, the sensor systems described herein are specifically designed to meet the needs of the oil recovery industry. Currently, there is no way for ships of opportunity to know the ratio of oil to water in skimming operations. The disclosed sensor systems of this present disclosure will solve that problem. The approaches described herein save operators money by reducing storage of seawater and increasing space for recovered oil. Furthermore, the approaches described herein will allow for fewer trips between recovery vessels and storage barges, and will further result in reduced cost for waste processing and filtering of seawater. These sensor systems will help improve efficiency and reduce the amount of seawater that must be processed, thereby reducing operating costs.

The systems and methods described herein may also be used in oil processing and as an oil cut sensor or water cut sensor to detect when wells are producing an unexpected amount of ground water.

The systems described herein have the ability to monitor oil-water percentages from 0% to 100% regardless of water salinity. In addition, the sensitivity of the sensor system is very uniform across the sensor cavity, making the sensor system independent of the oil-water dispersion, including mixtures that are not homogeneous, homogenous, or in an emulsion state.

The techniques described herein are suitably implemented in the form of one or more electronic processors executing instructions read from a non-transitory storage medium such as a hard drive or other magnetic storage medium, an optical disk or other optical storage medium, a cloud-based storage medium such as a RAID disk array, flash memory or other non-volatile electronic storage medium, or so forth. Some embodiments also include computers connected via an electronic network (e.g. WiFi, Ethernet, Internet, various combinations thereof, or so forth) to form a parallel computing resource, ad hoc cloud computing resource, or so forth.

The following examples are provided to illustrate the systems and methods of the present disclosure. The examples are merely illustrative and are not intended to limit the disclosure to the materials, conditions, or process parameters set forth therein.

EXAMPLES

Two sections of 3-inch schedule 40 acrylic pipes were connected to form a closed loop fluid recirculation system able to pump mixtures of oil and saline water, and served as the sensor cavity. One of the pipe sections was used as the sensing cavity. A propeller powered by a variable speed motor was inserted into the second pipe section and used to force flow through the sensor cavity. Two metal electrodes were attached to the outside of the sensory cavity pipe section on opposite sides of the pipe. These electrodes were connected with a capacitor (e.g., 10-47 pF) and an inductor (e.g., several turns, 0.4-inch diameter) to form a dielectric sensor as illustrated in FIG. 1.

FIG. 5 shows a typical response of the dielectric sensor for several mixtures of HYDROCAL 300 refined oil and 3 wt % solutions of Red Sea salt (mostly NaCl with some minerals characteristic of a marine environment), where oil+water equaled 100%. Each oil content mixture was tested at four values of mixer speed from 500 to 1,000 revolutions per minute (rpm). The minimum 500 rpm speed was chosen because it provided a uniform oil-water mixing without formation of small droplets or emulsion. The 1,000-rpm speed generated stable emulsions especially for oil content over 80%. The operating frequency was between 35 MHz and 41 MHz.

As it is clearly visible in FIG. 5, the resonance frequency versus oil content relation has two distinctive regimes. For mixtures above 70% oil content, the frequency is almost a linear function of oil content, as expected from the dielectric sensor principles. For these mixtures, the relation can be easily inversed thus allowing for calculation of oil content based on a measured resonance frequency. Importantly, for this part of the plot, the resonance frequency does not strongly depend on the mixer speed indicating low sensitivity to oil-water dispersion.

In contrast, mixtures with oil content below 70% generate very similar resonance frequency for all values of oil content with the exemption of data obtained at the lowest mixer speed of 500 rpm. This part of the frequency versus oil content cannot be reversed, meaning the oil content cannot be measured by the dielectric method for mixtures of this type. The response of the dielectric measurement shown in FIG. 5 does not appreciably improve with frequency. This was demonstrated in equivalent tests carried out at 90 MHz and 270 MHz that produced very similar results.

The two distinctive parts of data presented in FIG. 5 correlated with emulsion types observed during experiments. Oil-in-water mixtures or emulsions have much lower viscosity as compared to water-in-oil mixtures or emulsion. This difference of viscosity can be easily determined visually during experiments. All mixtures with oil content below 70% were low viscosity, therefore oil-in-water type. In contrast, all mixtures with oil content above 70% were viscous and of the water-in-oil type. Mixtures with 70% oil content can be of both types depending on how this oil content was achieved. Different oils behave differently when dispersed in water and the transition point between the two types of emulsions may be different, although it will generally occur between 30-80% oil content. Other factors like temperature, presence of dispersing additives or impurities, mixing conditions, and mixture history may also affect the transition point between the two mixture types. Finally, the transition is known to have significant hysteresis effect and not be very reproducible.

The eddy current measurement was tested for the same range of oil-water mixtures as used in the dielectric measurements. The main quantity recorded during the eddy currents test was the height of a resonance peak. The resonance frequency was also recorded, however, it remained constant (21.72 MHz) for all mixtures as well as for an empty pipe measurement. FIG. 6 shows the response of the eddy current sensor to oil mixtures with different oil content and mixer speeds.

The response of the eddy currents sensor was measured as the height of the resonance peak. The peak height increased monotonically with oil content up to 70% and was independent of the mixer speed. The peak height dependence on oil content was quite strong, considering that the decibel scale is logarithmic. Together, FIG. 5 and FIG. 6 indicate that the dielectric and the eddy current measurements are complementary and, if used in conjunction in one sensor system, can provide a reliable oil content measurement for the entire range of oil-water mixtures.

It should be noted that the resonance frequency of the dielectric sensor (FIG. 5) changed from about 36 MHz for the oil-in-water mixtures to about 39 MHz for the water-in-oil mixtures. Thus, for example, resonance frequency values between 37 MHz and 38 MHz could be used by the sensor system to determine whether to report the dielectric sensor measurement or the eddy current sensor measurement (see FIG. 3).

One important feature of the eddy current measurement is its dependence on water salinity. This is demonstrated in FIG. 7, which shows the eddy current signals obtained at 2 and 4 wt % water salinities.

The effectiveness of combining a dielectric sensor with an eddy current sensor was tested for several types of oil, both refined and crude, and for salinity levels 2%, 3%, and 5%. FIG. 8 shows the comparison between true oil fraction and the oil fraction obtained from the combined sensor system. The agreement was excellent across the entire range of oil-water mixtures. Average error of measurement was below 3%, and the maximum error was below 12% (both measured in units of oil fraction).

In addition, some embodiments, which are based on a combination of one dielectric sensor and one eddy current sensor, provide an accurate oil fraction information if the sensor cavity is fully filled with liquid oil-water mixture with no or only a minimum amount of air. However, if the tested stream contains a significant volume of air, or other gas or vapor, the sensor will interpret the gas volume as oil and effectively overestimate the oil fraction. This is particularly limiting for many types of oil recovery operations that produce streams containing a significant fraction of air and result in a “partially empty” condition in pipes and hoses used in these operations. A typical practice of oil recovery operations involves long and horizontal hoses or pipes and flow velocities that are too small to prevent flow stratification due to gravity. These conditions result in a stratified flow with oil-water mixture occupying the bottom part of the hose and air present on top.

An effective method to mitigate this adverse effect for sensors with streams containing air is to use a U-trap arrangement 900, as shown in FIG. 9. In this arrangement, the sensors 100, 200 are installed below the inlet 920 and outlet 925 of the U-trap arrangement 900, ensuring its cavity remains predominantly filled with liquids. In such an arrangement, passing air pockets are quickly displaced by buoyancy and spend relatively shorter time inside of the sensors 100, 200. This ensures that the effects of air are reduced, and the measured oil fraction is closer to its true value.

Embodiments relating to the U-trap method can be further improved if a second connection 910 (e.g. an air passage) is installed at the top of the main U-trap 905 to allow for air passage. This connection should be of smaller cross section since air has much lower viscosity as compared to oil or water. A smaller diameter connection will ensure relatively free flow of air but will restrict flow of liquids. For instance, for a 3- or 4-inch diameter sensor, and a ½ inch diameter air transfer connection may be used.

The example shown in FIG. 9 shows the U-trap design with the sensor mounted horizontally. However, this is not required. Any sensor orientation, including vertical, can be used. Still, the sensor mounted horizontally offers some benefits including: the overall height of the U-trap is minimized; the U-trap is less likely to trap solid impurities that may be present in the oil-water stream; and oil-water separation due to buoyancy is minimized.

The present disclosure has been described with reference to exemplary embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 

The invention claimed is:
 1. A system for measuring oil content of a fluid, comprising: a dielectric sensor, comprising: a first resonance circuit formed by a first capacitor, a first inductor, and a pair of electrodes adapted to be attached to an associated cavity through which the fluid can flow; and a first standing wave ratio (SWR) analyzer configured to measure a resonance frequency of the first resonance circuit; an eddy current sensor, comprising: a second resonance circuit formed by a second capacitor, and a second inductor configured to produce a magnetic field within the associated cavity; and a second SWR analyzer configured to measure a height of a peak of a resonance frequency of the second resonance circuit; and one or more processors configured to: determine if the oil content is above or below a threshold; if the oil content is above the threshold, report the oil content using the dielectric sensor; and if the oil content is below the threshold, report the oil content using the eddy current sensor.
 2. The system of claim 1, wherein the threshold is an oil content of 70 percent.
 3. The system of claim 1, wherein the threshold is an oil content corresponding to a resonant frequency that allows for distinction between oil-in-water mixtures and water-in-oil mixtures.
 4. The system of claim 1, wherein the threshold is set based on the oil content.
 5. The system of claim 1, wherein an operating frequency of the second SWR analyzer is selected based on: (i) a radius of a pipe of the sensor cavity, and (ii) a conductivity of the fluid.
 6. The system of claim 1, wherein the second inductor is a coil wound around a pipe of the sensor cavity.
 7. The system of claim 1, wherein no coil of the second inductor is in direct electrical contact with the fluid.
 8. The system of claim 1, wherein the one or more processors are further configured to make the determination of if the oil content is above or below the threshold by using the dielectric sensor.
 9. The system of claim 1, wherein the eddy current sensor is configured to account for water salinity by calibration of the sensor performed with water at a given salinity.
 10. The system of claim 1, wherein the eddy current sensor is configured to account for water salinity by manual entry of a salinity value.
 11. The system of claim 1, wherein the first inductor, the first capacitor, and the sensor cavity are all connected in parallel.
 12. The system of claim 1, wherein the first capacitor has a capacitance of about 10 pF to about 100 pF, and the first inductor has an inductance of about 0.1 μH to about 2 μH.
 13. The system of claim 1, wherein the second capacitor has a capacitance of about 50 pF to about 700 pF, and the second inductor has an inductance of about 0.1 μH to about 2 μH.
 14. The system of claim 1, further including a U-trap comprising an inlet and an outlet; wherein the dielectric sensor and the eddy current sensor are mounted in the U-trap: (i) in between the inlet and the outlet, and (ii) below the inlet and the outlet.
 15. The system of claim 14, wherein the system further includes an air passage positioned above the U-trap, and having a smaller diameter than a diameter of the U-trap.
 16. A method for measuring oil content of a fluid in a cavity, comprising: using a dielectric sensor, measuring the oil content of the fluid in the cavity; using one or more processors, determining that the oil content is below a threshold; and in response to the determination that the oil content is below the threshold, using an eddy current sensor to measure the oil content of the fluid.
 17. The method of claim 16, further comprising: using the one or more processors, determining that the oil content is above the threshold; and in response to the determination that the oil content is above the threshold, using the dielectric sensor to measure the oil content of the fluid.
 18. The method of claim 16, wherein the threshold is an oil content of 70 percent.
 19. The method of claim 16, wherein the threshold is an oil content corresponding to a resonant frequency that allows for distinction between oil-in-water mixtures and water-in-oil mixtures.
 20. The method of claim 16, wherein the eddy current sensor comprises: a resonance circuit formed by a capacitor, an inductor and the cavity, wherein the inductor is configured to expose the fluid within the cavity to a magnetic field; and a standing wave ratio (SWR) analyzer configured to measure a height of a peak of a resonance frequency of the resonance circuit.
 21. The method of claim 16, further comprising accounting for water salinity by calibrating the eddy current sensor using water at a given salinity.
 22. The method of claim 16, further comprising accounting for water salinity by manually entering a salinity value to the eddy current sensor.
 23. A system for measuring oil content of a fluid, comprising: a dielectric sensor, comprising: a first resonance circuit formed by a first capacitor, a first inductor, and a sensor cavity through which the fluid can flow; and a first standing wave ratio (SWR) analyzer configured to measure a resonance frequency of the first resonance circuit; an eddy current sensor, comprising: a second resonance circuit formed by a second capacitor, and a second inductor, wherein the second inductor is configured to expose fluid within the sensor cavity to a magnetic field; and a second SWR analyzer configured to measure a height of a peak of a resonance frequency of the second resonance circuit; at least one processor; and at least one memory including computer program code; wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the system to: determine if the oil content is above or below a threshold; if the oil content is above the threshold, measure the oil content using the dielectric sensor; and if the oil content is below the threshold, measure the oil content using the eddy current sensor. 